Trenchless laying of underground pipes

ABSTRACT

A method of laying lengths of underground pipe, for example drainage pipes, by forming an underground tunnel by means of a mole plough, and drawing a length of pipe into the tunnel into its required final position, characterized in that a liquid slurry is injected into the tunnel formed by the mole before the pipe length is drawn into the tunnel, the slurry serving to lubricate the pipe during its travel along the tunnel. Preferably the principal ingredient of the slurry is Bentonite. The pipe may be towed directly behind the mole and drawn progressively into the tunnel as it is formed or the pipe may be drawn into the slurry-filled tunnel after the mole plough has completed its traverse.

Unlted States Patent 1 1 3,589,135

[72] lnventor Ainsley Neville Ede 1,314,279 8/1919 McCrary 61/72.2 36Thornton Way, Cambridge, England 1,765,185 6/1930 Umbarger 61/72.2 X

[21] Appl. No. 806,912 1,904,666 4/1933 Sack 61/72.7

[221 Filed Mar. 13,1969 3,100,381 8/1963 Case et a1. 6l/50X [45]Patented June 29,1971 3,214,920 11/1965 Jacobs 61/72.7 X

[32] Pl'lOl'lly Mar. 15, 1968 FOREIGN PATENTS [33] Great Britain [32783/68 976,421 1964 Great Br1ta1n 6l/72.5

[S4] TRENCHLESS LAYING OF UNDERGRGUND PIPES Primary Examiner-JacobShapiro Attorney-watson, Cole, Grindle & Watson ABSTRACT: A method oflaying lengths of underground pipe, for example drainage pipes, byforming an underground tunnel by means of a mole plough, and drawing alength of pipe into the tunnel into its required final position,characterized in that a liquid slurry is injected into the tunnel formedby the mole before the pipe length is drawn into the tunnel, the slurryserving to lubricate the pipe during its travel along the tunnel.Preferably the principal ingredient of the slurry is Bentonite. The pipemay be towed directly behind the mole and drawn progressively into thetunnel as it is formed or the pipe may be drawn into the slurry-filledtunnel after the mole plough has completed its traverse.

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ATTORNEY PATENTED JUN29|97| 3,589,135

SHEET u' UF 5 vvvv Fla] INVENTOR ATTGRNEY IINVENTOR Amway NEVILLE EMATTORNEY TRENCHLESS LAYING OF UNDERGROUND PIPES This invention relatesto the laying of lengths of underground pipe, for example drainagepipes, in situ by forming an underground tunnel by means of a moleplough and drawing a length of pipe into the tunnel into its requiredfinal position. By this method, known as trenchless pipe laying, aconsiderable length of a pipe, perhaps a single length up to 350 feetlong and up to 18 inches or more in diameter, can be laid in positionunderground without the need for opening up a trench to receive thepipe. Either the underground tunnel is first formed by means of acomplete traverse of the mole plough and then a length of pipe is drawninto the preformed tunnel as by winching, or else the pipe length istowed behind e the advancing mole so as to be drawn progressively intothe tunnel as the latter is being formed by the traverse of the molethrough the ground.

A difficulty that has been encountered in the trenchless laying of pipearises from the very considerable soil friction which has to be overcomewhen drawing the pipe length into the newly formed tunnel, especiallywhere the tunnel lies at a considerable depth below ground, since thesoil friction is found to vary in almost direct proportion to the soildepth on account of the soil pressure due to compaction. It is an objectof the present invention to reduce this variation of soil friction withtunnel depth, and also to reduce the total soil friction encountered bythe pipe when being drawn into the tunnel; and in addition, when workingin loose earth or running sand or in waterlogged conditions, tostabilize the tunnel against collapse before the pipe is drawn into it,as well as to provide an improved and stabilized bed for the pipe.

According to the present invention, a liquid slurry of clay or cement orsoil, or an admixture of two or more of the same, with or without anyother ingredient or ingredients and similar to the slurries and mudscommonly used in the drilling of soil, is injected into the tunnelformed by the mole before the pipe length is drawn into the tunnel, theslurry serving to lubricate the pipe during its travel along the tunnel.It is found that the presence of the slurry in the tunnel isolates thepipe to a certain extent from direct contact with the tunnel wall, andnot only substantially reduces the frictional drag on the pipe but alsomakes this drag substantially independent of the depth of the tunnelbelow ground level.

Preferably the sole or a principal ingredient of the slurry isBentonite, which is a Montmorillonite clay in dried and ground formwhich will take up water in large quantities, up to 1,200 times its ownvolume, by a physicochemical effect. When mixed with water in suitablequantities Bentonite forms a jelly having thixotropic properties, whichcan be pumped down into the tunnel formed by the mole by means of apositive-displacement pump such as a worm-type pump or a Mono pump,either in metered quantities or in quantities limited only by the spaceavailable. Again, the slurry may be propelled down into the undergroundtunnel by pneumatic pressure, for instance applied to a closed hopperconnected to a delivery pipe leading down into the tunnel. In this casemetered delivery by means of a regulating valve is preferable, in casethe whole available quantity of slurry should otherwise be lost from thetunnel into a soil cavity.

The slurry instead of being simply a mixture of Bentonite and water maycomprise a mixture of Bentonite and soil in water, for example a mixtureof approximately 50:50 ratio of Bentonite and soild in water has beenfound satisfactory. Either the mixture may be premixed before delivery,or the Bentonite slurry may be delivered separately into the tunnel andthere mixed in place with the soil of the tunnel wall by means of anunderground mechanical mixer towed behind the mole.

Again, a mixture of Bentonite and cement in water may also be used asthe slurry, and will'set to provide a flrm bed for the pipe after thelatter has been drawn into position.

As an illustration of the effectiveness of the present invention, a350-foot length of 6-inch diameter bare pipe drawn into a bare soiltunnel formed by a mole plough might require a drag of 2,000 poundsweight at a depth ofa few inches below ground level, rising with asubstantially straight line characteristic to about 40,000 pounds weightat 10 feet depth. However if the tunnel contains a Bentonite slurry intowhich the pipe is drawn so that the slurry surrounds the circumferentialsurface of the advancing pipe and isolates it from direct contact withthe soil, the drag on the pipe may be expected to remain substantiallyconstant at about 12,000 pounds weight at all depths, at least up to 10feet.

The pipe-laying operation may be performed in either of two ways. Eithera complete length of the tunnel may first be formed by a traverse of themole plough, the slurry being pumped down through a passage in the moleplough coulter and delivered into the tunnel behind the mole so as tofill the tunnel completely, and when the mole plough has completed itstraverse a length of pipe may then be drawn into the slurryfilled tunnelby means of a cable and winch, or else the pipe length may be toweddirectly behind the advancing mole so that the pipe is drawnprogressively into the tunnel as the latter is being formed by the mole,the slurry being pumped via the mole plough coulter into the space: infront of the leading end ofthe pipe and around the circumference of thepipe.

In the former case, an expander of generally flaring hollow form may beattached at its smaller end preferably via a pivotal connection to thepipe-drawing cable immediately in front of the pipe length to expand thenewly formed passage before the main part of the pipe length enters it,and the slurry will be forced into the soil forming the circumferentialsurface of the tunnel by the expander, as well as passing around andpossibly through apertures in the expander to surround the ad vancingpipe length with a circumferential layer of slurry perhaps one-halfinchto 1 inch thick.

Preferably the expander is of such a form that the bottom of theexpander follows the path made by the bottom of the mole plough so thatexpansion occurs in the upwards and sideways directions which thus hasthe effect of completely sealing the channel above the passage fon-nedby the tool stalk of the mole plough.

The expander may be any suitable flared form, its leading end beingdimensioned to fit closely into the tunnel previously formed by themole, and it should be of symmetrical, substantially circular crosssection over its whole length, and it may be made free to rotate withoutdisturbance of its effective depth below ground level. For example theexpander may take the form of a simple frustum of a right cone, or abell, or series of cone frustra or bells of successively greatercross-sectional size, with or without intervening cylindrical sections.The expander may be of longitudinally fluted form instead of being ofpure circular section.

In practice it is desirable to use the same mole plough machine forlaying pipes of various sizes in which case the expander size is chosento suit the particular application. Conveniently the expander is formedof two portions the larger trailing end portion of the expander beingdetachable from the leading end portion, for example by a bayonettypeconnection, so that trailing end portions of different sizes can befitted according to the size of pipe to be laid.

The invention may be carried into practice in various ways, but onespecific embodiment will now be described by way of example only withreference tothe accompanying drawings, in which:

FIG. 1 is a sectional side view of a pipe-laying apparatus incorporatinga mole plough and an expander,

FIG. 2 is a detailed view showing the expander and pipe connectedthereto, as used in FIG. 1,

FIG. 3 is a perspective diagrammatic view of the trailermounted slurryreservoir to which the mole plough of FIG. 1 is connected,

FIG. 4 is a diagram showing schematically the valve and pump circuitassociated with the reservoir,

FIG. 5 is a perspective view of a modified form of expander, thetrailing end portion of which is detachable,

FIG. 6 is a view similar to FIG. I of a modified form of the apparatusincorporating the two-piece expander shown in FIG. 5,

FIG. 7 is a view similar to FIG. 1 of a modified form of the apparatusincorporating a rotary mixer, and

FIG. 8 is a view similar to FIG. 1 further modified to incorporate anexpander which is towed by a cable after the mole plough has made itsinitial tunnelling pass.

In the first embodiment, a mole plough 10 comprising a mole 11 ismounted at the foot of a coulter blade 12 having a rigid slurry feedpipe 13 attached generally vertically to its rear edge and leading atits lower end via a flexible connection to a rearwardly directed tube 14mounted on an expander 15 which is towed directly behind the moleplough, the tube 14 communicating with a passage 16 within the expander15 which comprises a simple, rigid, flared member of generallyfrustoconical external form and which is attached by means of a pivotallinkage 18 at its nose to the rear end of the mole 11, the pivotallinkage l8 permitting up and down and sideways pivotal movement of thetrailing expander 15. The expander 15 has a short extension ofcylindrical form 17 extending rearwardly from its rear end within whichis mounted the leading end of a continuous length of plastics pipe 24,the detachable manner by which the pipe 24 is connected to the expanderbeing shown in FIGS. 2 and 6.

As shown in FIG. 2 the leading end of the plastics pipes 24 is attachedby a pin 25 which passes through transverse holes in a short cylindricalspigot 26 located within the end of the pipe the spigot 26 being loosefitting in the bore of the pipe and being closed at its forward end, andhaving forwardly projecting lugs 27 to which are attached correspondinglugs 28 which cooperate with a releasable catch secured to the expanderIS. A plug 29 of foam rubber is compressed between the links and the endof the pipe 24 the purpose ofwhich is to prevent liquid slurry enteringthe pipe 24 or getting into the locking and release mechanism of thereleasable catch. The details of the releasable catch are shown in FIG.6 and will be described later.

The upper end of the feed pipe 13 attached to the coulter I2 isconnected by a hose 27 to a trailer-mounted reservoir 30 shown in FIGS.3 and 4 and provided with a positive-displacement pump 31 of the wormtype, to whose delivery outlet the hose 27 is connected.

The reservoir 30, the bottom part of which is of V-shaped cross section,is filled with a slurry of Bentonite and water in the form of athixotropic jelly 32, which can be pumped along the hose 27 and downthrough the slurry feed pipe 13 and then through the tube 14 and thepassage 16 in the expander 15 and around the foam plug 29 and into atunnel 33 newly formed in the soil by the traverse of the mole as shownin FIG. 2. As shown in FIGS. 3 and 4 the Bentonite slurry is pumped outfrom the lowest part of the V bottom of the reservoir 30 by the pump 31and then under pressure via a gate valve 34 to the delivery pipe 35which as shown in FIG. 3 communicates via a T connection with atransverse pipe 36 providing alternative delivery points to the left orto the'right below the reservoir frame. The hose 27 is connected to oneof the delivery points and a cap 37 is fitted to the delivery end whichis not in use.

A bypass return line 38 connects the outlet from the pump 31 back to thereservoir 30 via a gate valve 39 the return line 38 leading to a pipe 40leading through the interior of the reservoir 30 which pipe 40 withinthe reservoir has two pairs of small diameter forwardly directed jets 41for discharging the slurry back into the reservoir. A pressure controlvalve 42 is provided above the return line 38 in a second bypass line43, the pressure control valve being controlled by means of weights 44.

Initially for mixing the slurry, the reservoir 30 is filled with waterand the pump 31 started with the return line valve 39 open and thedelivery valve 34 closed so that all the pump delivery circulates backto the reservoir 30 via the return jets 4I. Bentonite powder is thenadded until the desired consistency is reached. With the weights 44suitably set, the return valve 39 is closed so that the circulation ofslurry is forced through the pressure control valve 42 back to thereservoir 30. The slurry can then be drawn off for delivery to the hose27 and expander 15 by opening the delivery valve 34.

The delivery pressure will be determined by the weights 44 of thepressure control valve 42, and the volume of slurry which is deliveredunderground, dependent upon the speed of the mole and thesize of thetunnel 3, is in practice always less than the capacity of the pump, theexcess slurry being returned via the pressure control valve 42 to thereservoir 30. The return jets 41 cause the recirculated slurry to stirthe bulk of the Bentonite slurry in the reservoir and being thixotropicthe slurry in the reservoir and being thixotropic the slurry thusremains liquid as long as it is circulated.

The reservoir 30 may be trailer mounted for drawing alongside the moleploughing machine or may instead be mounted on the mole plough machineitself.

In use the mole 11 and the rigid coulter blade 12 are forced through thesoil by means of a tractor or a winch to which the mole plough 10 iscoupled, with the mole running at a graded depth of perhaps 2 feet, toform a tunnel 33. The expander 15 and the leading end of the pipe 24attached thereto are towed directly behind the advancing mole 11, theexpander 15 expanding the tunnel by the upward and sideways compactionof the soil structure, thereby to close the channel above formed by thecoulter blade 12, the bottom wall of the tunnel remaining substantiallyundisplaced, by virtue of the expander pivotal connection 18 whichpermits angular freedom of the expander 15. Bentonite slurry iscontinuously pumped via the hose 27 and feed pipe 13 through theexpander 15, the slurry passing around the leading end of the pipe 24 tooccupy the annular space 38 between the circumference of the pipe 24 andthe wall of the expanded tunnel, so that the pipe length 24 will betowed behind the expander 15 through a layer of Bentonite slurry perhapsone-half inch thick at its minimum below the pipe, and somewhat thickerabove and around the sides of the pipe. This layer of slurry surroundingthe pipe 24 acts as a lubricant to reduce frictional drag as alreadydescribed.

In a modified form of the apparatus shown in FIGS. 5 and 6, a two-pieceexpander is used, the larger portion 51 of which at the trailing end isdetachable so that it can be interchanged for larger or smaller endportions according to the size of the pipe 24 to be laid. The detachabletrailing portion 51 of the expander can be fitted into the leading noseportion 52 by a bayonet type connection, by means of projecting lugs 53on a tubular shank 54 which is of common size for all expanders, whichlugs are inserted through slots 55 provided in the nose portion 52, thetrailing portion 51 then being rotated slightly and locked by means of asuitable locking bolt (not shown).

At the completion of a pipe-laying run it is convenient to release thepipe underground at a specific point without having to dig a hole aroundthe mole plough, and for this purpose a releasable catch is provided asshown in FIG. 6. The releasable catch comprises a pivotal hook mountedin the expander which engages a pin attached to the lugs 28 (see FIG. 2)connected to the end of the pipe 24, the hook 60 being locked in thetraction position by a spring loaded withdrawable bolt 61, the hook 60being spring biased to its released position by a spring 62. At the endof the run the hook 60 is opened by withdrawing the locking bolt 61 bypulling on a cable 63 attached thereto which cable passes around apulley 64 in the mole and up through the coulter to above ground level.A toggle 65 is provided on the hook 60 for use when attaching the end ofthe pipe 24 initially.

FIG. 7 shows a further modification of the apparatus incorporating amechanically driven rotary mixer mounted directly behind a rearwardlydirected nozzle 71 through which the slurry is discharged, the mixercomprising a power-rotated drum having radically protruding teeth bywhich the newly discharged slurry is mixed into the surface layer ofsoil forming the .wall of the tunnel. In this case the slurry 32 willpreferably comprise a mixture of Bentonite alone with water, no soilbeing present in the slurry or pumped down through the coulter. If nomechanical mixer is used, however, a proportion of soil may be includedin the slurry in the reservoir for pumping down into the tunnel.

In a modified engagement arrangement and process, illustrated in FIG. 8the mole plough first makes an initial tunnelling pass during which theexpander and pipe 24 are left at the entry point of the tunnel 33 andthereafter the expander and pipe 24 are winched along the tunnel 33.

In this case the coulter blade 80 has an internal duct 81 leading to arearwardly directed circular nozzle 82, and the expander 83 comprises asimple rigid, flared tubular sheet metal member of frustoconical form,about 8 feet long, which is formed with a series of apertures 84 in itswall and a central aperture 85 in its nose, is attached by means of aswivel joint 86 at its nose, is attached by means of a swivel joint 86at its nose to the rear end of a towing cable 19 associated with themole plough. The rear diameter of the mole 83 is 8 inches and thediameter of the leading end of the expander is about the same; at itsrear end the expander is 24 inches in diameter.

Attached to the end of a towing cable 88 (which may convenientlycomprise the same cable as that used for releasing the catch 60 shown inFIG. 6 if the same mole plough is used for this embodiment) is theleading end of a pipe towing rod 89. At its rear end the towing rod 89is secured by a quickrelease coupling 90 to the leading end of acontinuous length of pipe 24 some 350 feet in length and 18 inches indiameter. The towing rod 89 extends through the interior of the conicalexpander 83 from its leading end, and the quick-release coupling 90 islocated just inside the rear end of the expander 83. The towing cable 88leads forwardly from the expander 83 to a pulley 91 in the base of thehollow coulter blade 80 of the mole plough, whence it is passed upwardlythrough the coulter and around a second pulley 92 above ground level toa winch (not shown), the winch having a free spooling drum on which thecable is stored.

In use the mole makes an initially tunnelling pass with the mole runningat a graded depth of perhaps 2 feet, to form a tunnel 33 initially 8inches in diameter, which is filled with Bentonite slurry 32 pumped downthrough the coulter passage 81 as the tunnel is being formed by thetraverse of the mole. The expander 83 and pipe 24 will be left at theentry point of the tunnel 33, and as the mole plough advances thefree-spooling drum of the cable winch pays out cable 92 which passesdown the coulter blade 80. When the mole plough reaches the forward endof the required pipe run it is stopped and the full power of the tractoror winch motor is applied to the cable winch to wind in the pipe-towingcable 92 and draw the expander 83 and the pipe length 24 through theslurry-filled underground tunnel 33 into the final required position ofthe pipe length. This pipe-drawing operation is performed very quickly,and as the expander 83 is drawn along the slurry-filled tunnel 33 itwill expand the tunnel to about 24 inches diameter, forcing some slurry32 into the tunnel wall. The expansion of the tunnel will again beeffected by the upward and sideways compaction of the slurry-impregnatedsoil structure, and the pipe 24 when drawn into the slurry-filled tunnel33 behind the mole 11 will rest directly on this undisturbedslurryimpregnated bottom of the tunnel in its predetermined grad ing.The slurry 32 previously pumped into the interior of the tunnel 33 infront of the advancing towed expander 83 will pass around and throughthe nose aperture 85 of the expander and will occupy the annular spacebetween the circumference of the pipe 24 and the wall of the expandedtunnel.

In this process, either the correct amount of Bentonite slurry 32required to fill the annular space around the pipe 24 is metered anddelivered into the tunnel 33 before the pipe is towed into it, and thereturn of slurry through the coulter passage 81 during pipe towing isprevented by the closure if the stop valve 34; or else the tunnel 33 isfirst completely filled with the slurry, and the pipe 24 is then towedinto the filled tunnel and excess slurry is forced back up through theconduit 81 and the hose 27 back into the reservoir 30 by the backpressure due to the advancing pipe and expander, the stop valve 34 beingopen and the pressure'loaded relief valve 42 operating to permit andcontrol this back flow.

The use of the expander 83 facilitates the maintenance of the grade ofthe pipe 24 by utilizing the undisturbed bottom surface of the tunnel 33as the reference line for the grade, despite the considerable distancebetween the entry point for pipe drawing and the foot of the mole ploughwhich previously determined this reference line.

Whilst, as described above and illustrated, the attachment of the pipetow rod 89 is to the main towing cable 92, and the expander 83 is freeto pivot in any direction independently of the pipe 24, it is alsopossible, if required, to utilize the pipe draught tension to impose asmall biassing force on the expander 33 towards its centralizedposition. This may be done by attaching the leading end of the pipe towrod 89 rigidly to the leading end of the expander 83 itself, for exampleby means of a spider or other suitable fitting, and to provide a swiveljoint in the pipe tow rod 89 a short distance, say 2 inches, behind theconnection to the expander.

What I claim as my invention and desire to secure by Letters Patent is:

1. A method of laying a length of underground pipe in situ, whichcomprises forming a generally horizontal underground tunnel by forcingthe mole of a mole plough through the ground, towing an expander behindthe advancing mole to ex pand the cross section of the undergroundtunnel, and towing a length of pipe behind the expander along theexpanded tunnel into its required final position, the pipe being ofsmaller cross section than the expander so that an annular-sectionclearance is defined between the exterior of the pipe length and theinterior surface of the expander tunnel, and simultaneously extruding athixotropic slurry of Bentonite and water under positive pressurethrough an extrusion orifice in the advancing expander and into the saidannular clearance around the advancing pipe length to fill the saidclearance with an annular layer of said thixotropic slurry through whichthe pipe length advances behind the expander as said tunnel isprogressively formed and expanded, the slurry being pumped to theextrusion orifice by means of a positive-displacement pump from areservoir above ground through a duct extending down to the expander.

2. A method as claimed in claim 1, in which an expander of generallyflared form is attached at its smaller end to the mole immediately infront of the pipe length and is towed through thetunnel with the pipelength so as to expand the tunnel before the main part of the pipelength enters it.

3. A method as claimed in claim 2 wherein the leading end of the pipe isconnected to the expander by means ofa releasable catch, which can bereleased by releasing means located above ground level, whereby at theend of the pipe-laying run the pipe can be detached from the mole ploughcoulter whilst underground.

4. A method of laying lengths of underground pipe by forming anunderground tunnel by means of a mole plough, and drawing a length ofpipe into the tunnel into its required final position, characterized inthat a liquid slurry is injected into the tunnel formed by the molebefore the pipe length is drawn into the tunnel, the slurry serving tolubricate the pipe during its travel along the tunnel, the principalingredient of the slurry being Bentonite, the slurry comprising amixture of Bentonite and soil in water, forming a gel having thixotropicproperties, and in which a mixture of Bentonite and water is deliveredseparately into the tunnel by a positive displacement pump and is theremixed in place with the soil of the tunnel wall by means of anunderground mechanical mixer towed behind the mole.

5. A method of laying a length of underground pipe in situ whichcomprises forming a generally horizontal underground tunnel ofappreciably greater cross-sectional area than the pipe to be laidtherein by forcing the mole of a mole plough through the ground, whilstsimultaneously extruding a thixotropic slurry of Bentonite and waterunder positive pressure into the tunnel through an extrusion orifice atthe rear of the advancing mole to fill the tunnel completely withpressurized slurry as the tunnel is progressively formed behind theadvancing mole, and, subsequently, drawing a length of pipe by means ofa cable and winch along the length of the preformed tunnel and throughthe filling of slurry therein into the required final position of thepipe length, the pipe length being drawn lengthwise through the fillingof slurry which forms an annular layer of slurry surrounding the pipelength and a smaller end to the pipe-drawing cable immediately in frontof the pipe length and is towed through the tunnel with the pipe lengthso as to expand the newly formed and slurry-filled tunnel before themain part of the pipe length enters it.

1. A method of laying a length of underground pipe in situ, whichcomprises forming a generally horizontal underground tunnel by forcingthe mole of a mole plough through the ground, towing an expander behindthe advancing mole to expand the cross section of the undergroundtunnel, and towing a length of pipe behind the expander along theexpanded tunnel into its required final position, the pipe being ofsmaller cross section than the expander so that an annular-sectionclearance is defined between the exterior of the pipe length and theinterior surface of the expander tunnel, and simultaneously extruding athixotropic slurry of Bentonite and water under positive pressurethrough an extrusion orifice in the advancing expander and into the saidannular clearance around the advancing pipe length to fill the saidclearance with an annular layer of said thixotropic slurry through whichthe pipe length advances behind the expander as said tunnel isprogressively formed and expanded, the slurry being pumped to theextrusion orifice by means of a positivedisplacement pump from areservoir above ground through a duct extending down to the expander. 2.A method as claimed in claim 1, in which an expander of generally flaredform is attached at its smaller end to the mole immediately in front ofthe pipe length and is towed through the tunnel with the pipe length soas to expand the tunnel before the main part of the pipe length entersit.
 3. A method as claimed in claim 2 wherein the leading end of thepipe is connected to the expander by means of a releasable catch, whichcan be released by releasing means located above ground level, wherebyat the end of the pipe-laying run the pipe can be detached from the moleplough coulter whilst underground.
 4. A method of laying lengths ofunderground pipe by forming an underground tunnel by means of a moleplough, and drawing a length of pipe into the tunnel into its requiredfinal position, characterized in that a liquid slurry is injected intothe tunnel formed by the mole before the pipe length is drawn into thetunnel, the slurry serving to lubricate the pipe during its travel alongthe tunnel, the principal ingredient of the slurry being Bentonite, theslurry comprising a mixture of Bentonite and soil in water, forming agel having thixotropic properties, and in which a mixture of Bentoniteand water is delivered separately into the tunnel by a positivedisplacement pump and is there mixed in place with the soil of thetunnel wall by means of an underground mechanical mixer towed behind themole.
 5. A method of laying a length of underground pipe in situ whichcomprises forming a generally horizontal underground tunnel ofappreciably greater cross-sectional area than the pipe to be laidtherein by forcing the mole of a mole plough through the ground, whilstsimultaneously extruding a thixotropic slurry of Bentonite and waterunder positive pressure into the tunnel through an extrusion orifice atthe rear of the advancing mole to fill the tunnel completely withpressurized slurry as the tunnel is progressively formed behind theadvancing mole, and, subsequently, drawing a length of pipe by means ofa cable and winch along the length of the preformed tunnel and throughthe filling of slurry therein into the required final position of thepipe length, the pipe length being drawn lengthwise through the fillingof slurry which forms an annular layer of slurry surrounding the pipelength and separating it from the wall of the tunnel as the pipe lengthadvances.
 6. A method as claimed in claim 5, in which an expander ofgenerally flared form and provided with at least one passage for theflow of slurry therethrough is pivotally attached at its smaller end tothe pipe-drawing cable immediately in front of the pipe length and istowed through the tunnel with the pipe length so as to expand the newlyformed and slurry-filled tunnel befoRe the main part of the pipe lengthenters it.